Propylene glycol and ethylene glycol are produced from petrochemical sources. Commercial production of propylene glycol involves the hydration of propylene oxide, which is made by the oxidation of propylene. The commercial production of ethylene glycol involves the hydration of ethylene oxide, made by the oxidation of ethylene. Propylene and ethylene are industrial by-products of gasoline manufacture, for example as by-products of fluid cracking of gas oils or steam cracking of hydrocarbons.
The world's supply of petroleum is being depleted at an increasing rate. Eventually, demand for petrochemical derived products will outstrip the supply of available petroleum. When this occurs, the market price of petroleum and, consequently, petroleum derived products will likely increase, making products derived from petroleum more expensive and less desirable. As the available supply of petroleum decreases, alternative sources and, in particular, renewable sources of comparable products will necessarily have to be developed. One potential renewable source of petroleum derived products is products derived from bio-based matter, such as agricultural and forestry products. Use of bio-based products may potentially counteract, at least in part, the problems associated with depletion of the petroleum supply.
In an effort to diminish dependence on petroleum products the United States government enacted the Farm Security and Rural Investment Act of 2002, section 9002 (7 U.S.C. 8102), hereinafter “FRISA”, which requires federal agencies to purchase bio-based products for all items costing over $10,000. In response, the United States Department of Agriculture (“USDA”) has developed Guidelines for Designating Bio-based Products for Federal Procurement (7 C.F.R. §2902) to implement FRISA, including the labeling of bio-based products with a “U.S.D.A. Certified Bio-based Product” label.
As used herein, the term “bio-derived” refers to a product derived from or synthesized by a renewable biological feedstock, such as, for example, an agricultural, forestry, plant, bacterial, or animal feedstock. As used herein, the term “bio-based” refers to a product that includes in whole or in significant part, biological products or renewable agricultural materials (including, but not limited to, plant, animal and marine materials) or forestry materials. As used herein, the term “petroleum derived” refers to a product derived from or synthesized from petroleum or a petrochemical feedstock. Propylene glycol that is produced by hydrogenolysis of a polyol, such as a carbohydrate, is referred to as bio-based propylene glycol.
FRISA has established certification requirements for determining bio-based content. These methods require the measurement of variations in isotopic abundance between bio-based products and petroleum derived products, for example, by liquid scintillation counting, accelerator mass spectrometry, or high precision isotope ratio mass spectrometry. Isotopic ratios of the isotopes of carbon, such as the 13C/12C carbon isotopic ratio or the 14C/12C carbon isotopic ratio, can be determined using isotope ratio mass spectrometry with a high degree of precision. Studies have shown that isotopic fractionation due to physiological processes, such as, for example, CO2 transport within plants during photosynthesis, leads to specific isotopic ratios in natural or bio-derived compounds. Petroleum and petroleum derived products have a different 13C/12C carbon isotopic ratio due to different chemical processes and isotopic fractionation during the generation of petroleum. In addition, radioactive decay of the unstable 14C carbon radioisotope leads to different isotope ratios in bio-based products compared to petroleum products. Bio-based content of a product may be verified by ASTM International Radioisotope Standard Method D 6866. ASTM International Radioisotope Standard Method D 6866 determines bio-based content of a material based on the amount of bio-based carbon in the material or product as a percent of the weight (mass) of the total organic carbon in the material or product. Bio-derived and bio-based products will have a carbon isotope ratio characteristic of a biologically derived composition.
Biology offers an attractive alternative for industrial manufacturers looking to reduce or replace their reliance on petrochemicals and petroleum derived products. The replacement of petrochemicals and petroleum derived products with products and/or feed stocks derived from biological sources (i.e., bio-based products) offer many advantages. For example, products and feed stocks from biological sources are typically a renewable resource. As the supply of easily extracted petrochemicals continues to be depleted, the economics of petrochemical production will likely force the cost of the petrochemicals and petroleum derived products to higher prices compared to bio-based products. In addition, companies may benefit from the marketing advantages associated with bio-derived products from renewable resources in the view of a public becoming more concerned with the supply of petrochemicals.
A number of commercial processes which produce polyols from complex mixtures of carbohydrates exist. These processes usually produce a homologous series of glycols. Some of the resulting polyols boil so close to one another that separation of the polyols by ordinary rectification is difficult. The relative volatility is so low that a large number of theoretical plates are required to produce high purity polyols.
In a process involving hydrocracking of higher carbohydrates, such as glucose, sorbitol or sucrose, the molecule is broken into fragments of lower molecular weight to form compounds which belong to the glycol or polyol family. For instance, U.S. Pat. No. 5,206,927 describes a homogeneous process for hydrocracking carbohydrates in the presence of soluble, transition metal catalyst with the production of lower polyhydric alcohols. A carbohydrate is contacted with hydrogen in the presence of a soluble transition metal catalyst and a strong base at a temperature of from about 25° C. to about 200° C. and a pressure of from about 15 to about 3000 psi. However, as is evident from Tables II and III in the disclosure of U.S. Pat. No. 5,206,927, about 2-7% of other polyol compounds are produced in the hydrocracking process.
U.S. Pat. Nos. 5,276,181 and 5,214,219 describe a process of hydrogenolysis of glycerol using copper and zinc catalyst in addition to sulfided ruthenium catalyst at a pressure over 2100 psi and temperature between 240-270° C. U.S. Pat. No. 5,616,817 describes a process of preparing 1,2 propanediol by catalytic hydrogenolysis of glycerol at elevated temperature and pressure using a catalyst comprising the metals cobalt, copper, manganese and molybdenum. German patent DE 541362 describes the hydrogenolysis of glycerol with a Nickel catalyst, while U.S. Pat. No. 4,476,331 describes a two stage method of hydrocracking carbohydrates (for example glucose), wherein a modified ruthenium catalyst is used for hydrocracking sorbitol to produce glycerol derivatives. European Patent applications EP-A-0523 014 and EP-A-0 415 202 describe a process for preparing lower polyhydric alcohols by catalytic hydrocracking of aqueous sucrose solutions at elevated temperature and pressure using a catalyst whose active material comprises the metals cobalt, copper and manganese. Persoa & Tundo (Ind. Eng. Chem. Res. 2005, 8535-8537) describe a process for converting glycerol to 1,2-propanediol by heating under low hydrogen pressure in presence of Raney nickel and a liquid phosphonium salt. Selectivities toward 1,2-Propanediol as high as 93% were reported, but required using a pure glycerol and long reaction times (20 hrs). Crabtree et al. (Hydrocarbon processing February 2006 pp 87-92) describe a phosphine/precious metal salt catalyst that permit a homogenous catalyst system for converting glycerol into 1,2-PD. However, low selectivity (20-30%) was reported. Other reports indicate use of Raney Copper (Montassier et al. Bull. Soc. Chim. Fr. 2 1989 148; Stud. Surf. Sci. Catal. 41 1988 165), copper on carbon (Montassier et al. J. Appl. Catal. A 121 1995 231)), copper-platinum and copper ruthenium (Montassier et al. J. Mol. Catal. 70 1991 65). Other homogenous catalyst systems such as tungsten and Group VIII metal-containing catalyst compositions have been also tried (U.S. Pat. No. 4,642,394). Miyazawa et al. (J. Catal. 240 2006 213-221) & Kusunoki et al (Catal. Comm. 62005 645-649) describe a Ru/C and ion exchange resin for conversion of glycerol in aqueous solution. Again their process however, results in low conversions of glycerol (0.9-12.9%).
One of the problems of producing glycerol derivatives by hydrogenolysis of glycerol is that other diol compounds are formed which reduce the purity of the desired component. For instance, in hydrocracking of higher carbohydrates such as, for example, sorbitol to produce propylene glycol, typically 3-5% by weight of 2,3-butanediol is produced in addition to 1,2 butanediol, ethylene glycol and 1,3-butanediol. These products are referred to as “polyols” or “polyhydric alcohols”. The boiling points of these components as shown in Table 1 are very close to one another such that in a rectification column, either under atmospheric, reduced pressure or at an elevated pressure, the separation of substantially pure propylene glycol from these other polyhydric alcohols is difficult to be attained.
TABLE 1Polyols produced by Hydrocracking of SorbitolPolyolWeight PercentBoiling Point, ° C.2,3-Butanediol3.5182Propylene glycol16.51871,2-Butanediol2.0192Ethylene glycol25.21981,3-Butanediol2.72062,3-Hexanediol—2061,2-Pentanediol—2101,4-Pentanediol—2201,4-Butanediol2.12301,5-Pentanediol0.1242Diethylene glycol2.22451,6-Hexanediol—250Triethylene glycol2.1285Glycerin38.82901,2,4-Butanetriol4.8190/18 mm100.00
The differences in volatility of propylene glycol compared to 2,3-butanediol or 1,2 butanediol are very small. As shown in Tables 2 and 3, the number of plates required to achieve 99% purity is very large, requiring the use of very tall distillation columns (55 trays for 2,3-Butanediol and 88 trays for 1,2-Butanediol) and high energy inputs.
TABLE 2Theoretical and Actual Plates Required vs. Relative volatility forSeparation of Propylene Glycol and 2,3-Butanediol.Actual Plates, 75%Relative VolatilityTheoretical PlatesEfficiency1.2541551.3531421.4525341.5023311.701824
TABLE 3Theoretical and Actual Plates Required vs. Relative volatility forSeparation of Propylene Glycol and 1,2-Butanediol.Actual Plates, 75%Relative VolatilityTheoretical PlatesEfficiency1.1566881.523312.014193.09123.5811
Several reports in the literature describe efforts for azeotropically separating glycerol derivatives such as 2,3 butanediol from propylene glycol. For instance, U.S. Pat. No. 4,935,102 describes a method for using an azeotrope forming agent such as propylene glycol isobutyl ether, tetrahydro furfuryl alcohol, N,N-dimethylacetamide, ethylene glycol diethyl ether, diethylene glycol diethyl ether, 2-methoxyethyl ether, ethylene glycol n-butyl ether, diacetone alcohol and ethyl n-butyl ketone. In U.S. Pat. No. 5,423,955, the azeotrope forming agent consists of a material selected from the group consisting of toluene, ethyl benzene, o-xylene, p-xylene, cumene, m-diisopropyl benzene, m-diethyl benzene, mesitylene, p-cymene, hexane, cyclohexane, methyl cyclohexane, heptane, 3-methyl pentane, octane, decane, 2,3,4-trimethyl pentane, dipentene, decalin, dicyclopentadiene, alpha-phellandrene, limonene, hemimellitene, myrcene, terpinolene, p-mentha-1,5-diene, beta-pinene, 3-carene, 1-heptene, cyclopentane, pentane, o-diethyl benzene, 2,2-dimethyl butane and 2-methyl butane. The azeotrope forming agents described in U.S. Pat. Nos. 4,935,102 and 5,423,955 may be characterized by their Hansen solubility parameters (Table 4).
TABLE 4Azeotropic agents used for separation of 2,3-Butanediol from propyleneglycol (U.S. Pat. No. 4,935,102).Azeotropic agentHansen pHansen hPropylene glycol isobutyl ether5.4212.52Tetrahydrofurfuryl alcohol10.4610.96N,N-dimethylacetamide11.4710.23Toluene0.751.98Ethyl benzene0.651.85p-Xylene0.911.84m-Xylene0.911.84o-Xylene0.911.84Cumene0.581.74Mesitylene0.981.7Ethylene glycol diethyl ether9.1914.3Diethylene glycol diethyl ether9.2212.332-Methoxyethyl ether1.817.41Ethylene glycol-n-butyl ether5.1312.27Diacetone alcohol8.1710.763-heptanone5.283.93
Azeotropic distillation using organic solvents as an azeotropic agent has also proven useful for azeotropically separating ethylene glycol from 1,2 butanediol (Table 5).
TABLE 5Azeotropic agents used for separation of 1,2-Butanediol from ethyleneglycol (U.S. Pat. No. 5,432,955).Azeotropic agentHansen pHansen h3-Heptanone5.283.93Cyclohexanone3.135.08Diisobutyl ketone4.93.79Methyl isoamyl ketone6.034.2Isobutyl heptyl ketone3.763.312-Methoxyethyl ether1.817.412,6-Dimethyl-4-heptanone4.903.79p-Xylene0.911.84m-Xylene0.911.84o-Xylene0.911.84Ethyl benzene0.651.85Cumene0.581.74Mesitylene0.981.7
The azeotropic agents used in U.S. Pat. Nos. 4,935,102 and 5,432,955 can be described by Hansen solubility parameters, which are described in detail in “Hansen Solubility Parameters: A User's Handbook,” by Charles M. Hansen (CRC Press, 1999), which is incorporated by reference in its entirety. Hansen solubility parameters can be calculated using the program “Molecular Modeling Pro Plus (version 6.0.6, Norgwyn Montgomery Software Inc, available from ChemSW, Inc) based on values published in the “Handbook of Solubility Parameters and Other Parameters” by Allen F. M. Barton (CRC Press, 1983) for solvents obtained experimentally by Hansen. The Hansen “h” (hydrogen bonding) values at 25° C. and Hansen “p” (polarity) values ° C. listed in Tables 4 and 5 were calculated in this manner.
Thus, a need exists for an economical process of separating polyethylene glycol and/or ethylene glycol from other polyhydric alcohols.